Method for Improving an Inwards Stability of a Levee

ABSTRACT

A method for improving an inwards stability of an existing levee including a landslide slope ( 128 ) and at least one of a landside berm ( 132 ), a landside heel, and a landside trench. The method comprises placing columns ( 160 ) through the landside berm into one or more soft soil layers ( 40 ) with corresponding soil volumetric weights (γs), wherein the columns have a column volumetric weight (γc) that is at least 10% larger than the soil volumetric weights, and wherein the columns comprise a mineral aggregate and an impermeable filler.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from Europeanapplication number 15176528.6 filed on 13 Jul. 2015, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The invention relates to a method for improving an inwards stability ofa levee.

BACKGROUND ART

A “levee” (also referred to as a “dike”) is a flood bank that protectsthe nearby lowland against flooding. Levees can be roughly classifiedinto primary levees comprising of sea-dikes, river-dikes, and inlandwater dikes and secondary levees (situated within the primary dikering).

A general overview of conventional terminology used for describinglevees is presented below, with reference to FIG. 1. Cartesiancoordinates are used to define directions and spatial properties oftypical levee elements. Reference symbol X is used to indicate alongitudinal direction. For a river-dike, the longitudinal direction Xtypically corresponds to the macroscopic flow direction of the river.Reference symbol Y is used to indicate a transversal directionperpendicular to the longitudinal direction X. The longitudinaldirection X and the transversal direction Y span a virtual plane that ispredominantly parallel with the horizontal (which macroscopicallycorresponds with the earth surface and is directed perpendicular to thedirection of gravity). Reference symbol Z is used to indicate a heightdirection (vertical direction), which is perpendicular to thelongitudinal direction X and the transversal direction Y. Theprepositions “above” and “below” pertain to the height direction Z.

FIG. 1 shows a side view of a levee 10 in a cross-sectional profilealong the transverse and vertical directions Y, Z. The levee 10 andtypical elements shown in FIG. 1 may be present in river-dikes, inlandwater dikes, as well as sea dikes. Locations and dimensions of theelements may vary in each levee type.

FIG. 1 shows that the foundation below and near the levee 10 may bepartitioned into a waterside land 16, a lowland 18, and a levee base 20.On a water side 12 of the levee 10, a body of water 35 (e.g. river,lake, or sea) is situated along a waterside land 16. For a river-dike,this water body 35 is formed by a river with a macroscopicflow-direction along the longitudinal direction X. The lowland 18 issituated on a land side 14 of the levee 10. A dry patch of land issituated at/along the lowland 18. Various structures and vegetation(e.g. roads, buildings, trees, etc.) may be present in this region (notshown in FIG. 1).

As shown in FIG. 1, the levee 10 is situated on a levee base 20, andcomprises a levee core 22, a levee crown 24, a waterside slope 26, alandside slope 28, a waterside berm 30 with toe 31, and a landslide berm32 with a heel 33 and a trench 36. It should be understood that in otherlevee arrangements, the trench and landside berm do not necessarily haveto be situated directly near the toe.

The levee base 20 corresponds with the subsoil below the levee 10. Thelevee core 22 forms the supporting construction onto which the variouselements of the levee 10 are situated. The levee core 22 must remainstable under the total load from the various levee elements, as well asthe (variable) load of the water and traffic present on the levee 10.

The crown 24 defines the apex of the levee structure. The height of thecrown 24 has a significant impact on the probability of overtopping andoverflowing if the levee 10 is subjected to changing water levels (e.g.resulting from wave action). A road, other structures, and/or vegetationmay also be present on the crown 24 (not shown in FIG. 1).

The waterside slope 26 defines a slanting surface on the water side 12of the levee 10. The landside slope 28 defines another slanting surfaceon the land side 14 of the levee 10. The waterside and landside slopes26, 28, and in particular their respective tilting angles, are animportant factor for the levee's stability.

The waterside berm 30 may be present in particular in sea-dikes andinland water dikes, to reduce overtopping by waves and thus improve thedamming efficiency. The landside berm 32 may be present to increase thestability of the landside slope 28. The waterside toe 31 forms thewaterside base perimeter of the levee 10. The landside heel 33 forms thelandside base perimeter of the levee 10. A drainage trench 36 isprovided along the landside heel 33 of the levee 10, arranged fordewatering the levee. Sometimes, drainage elements are provided in apart of the levee core 22 or the berm 32, which may be fluidly connectedto the trench 36.

The levee elements 22-33 and the subsoil elements 16-20 are typicallyformed by soil layers of different constitution and typical massdensities, like clay, peat, and/or sand. For levees, the hydrologicalproperties and the behavior of the soil layers in response to changingwater conditions are of great impact on the stability of the levee 10and the hydraulic load (ground water) on the levee 10. This situationgreatly differs from that of purely land-based embankments (e.g. whichmay be employed as a base for supporting roads and train tracks). As aconsequence of high water conditions, the ground water level in thelevee core 22 will rise and is affected by the permeability of the toplayer, the subsoil 20, the core 22 and the berm 32. Deeper sand layersin the subsoil 20 can develop high hydraulic loads, affecting thestability of non permeable top layers at the inland 18 and berm 32 andtrench 36. This affects failure mechanisms of the levee 10.

If the structural integrity of a levee (e.g. levee 10 in FIG. 1) isinsufficient to resist the influence of the water 35, then the levee mayneed to be rebuilt. In view of cost and logistic considerations, it ispreferable to reinforce an existing levee instead of breaking it downand building a new levee from scratch.

A levee needs to be resistant to various failure mechanisms. Two suchfailure mechanisms relate to macro (in)stability and piping.

The term “macro stability” (of a levee) corresponds to the ability ofthe levee to resist sliding of the slopes along slip surfaces extendingthrough the core and the base of the levee. Such sliding faults mayoccur along planar slip surfaces as well as along curved slip surfaces(e.g. cylindrical shape, piecewise curved, etc.). Here, the term“surface” should not be construed narrowly as a plane or an outersurface, but be generally construed as a surface in the geometricalsense (i.e. a geometric entity that requires a two dimensional parameterdomain for defining a thin continuous object extending in threedimensional space). Such a slip surface describes the soil region alongwhich the sliding faults may occur. Levee properties that affect themacro stability are e.g. the geometry of the levee elements, thelocation of the various soil layers in/below the levee, the massdensities and mechanical properties of these soil layers (e.g. shearstrength, compressibility, etc), as well as the dynamic external loadsacting on the levee (e.g. resulting from water dynamics, weatherinfluences, and potential traffic and structures on the levee) andgroundwater conditions affected by water dynamics.

The term “piping” relates to seepage of ground water though the sandlayer(s) underneath the levee, usually in the landside direction. Suchsand layers are typically relatively permeable, and the eroding effectof this seepage creates local conduits that extend through these sandlayers underneath the landside slope 28 and berm 32. Such conduits,which may for example emanate in the trench 36, will weaken the levee inlocations that are relatively hard to predict.

A known method for improving the macro stability of an existing leveeinvolves the construction of a support berm made of natural materials,pre-processed materials, and/or materials recycled from industry.Construction of such a support berm in an existing levee often requiresthe existing trench to be moved towards the inland, in order to createspace for the support berm. It is also known that a levee may bereinforced by introducing rigid partitioning walls into various regionsof the levee, to increase stability and also water impermeability of thelevee. These walls affect the permeability of the levee and the subsoil.

A more recent method for reinforcing a levee is known as the so-called“mixed-in-place” (MIP) method. In this method, selected portions of thesoil layers inside the levee are mixed in situ with cement to form mixedsoil columns. These soil columns are mixed into the levee core, startingfrom the crown or the landside slope, and extend in a tilted directiondownwards to the supporting sand layer located below the levee. The MIPmethod involves the formation of rows of overlapping columns (“panels”)that extend in the transversal direction perpendicular to the levee. Byjoining several column panels, blocks of stabilized ground are formedwhich are mutually separated by batches of unmixed soil.

The MIP method is relatively labor intensive. It would therefore bedesirable to provide a method for improving the inwards stability of alevee using a relatively simple non-invasive procedure.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method forimproving an inwards stability of an existing levee including alandslide slope and at least one of a landside berm, a landside heel,and a landside trench, wherein the method comprises placing columnsthrough at least one of the landside berm, the landside heel, and thelandside trench, into one or more soft soil layers with correspondingsoil volumetric weights, wherein the columns have a column volumetricweight that is at least 10% larger than the soil volumetric weights, andwherein the columns comprise a mineral aggregate and an impermeablefiller.

The columns serve as structures for increasing the average mass densityof the penetrated soil, and for increasing the total mass of at leastone of the landside berm region, the landside heel region, and thelandside trench region of the levee. The resulting mass increase of thelandside berm and/or heel and/or trench region contributes to thestability of inwards sliding surfaces and reduces to probability ofheave or hydraulic lift of this landside berm and/or heel and/or trenchregion, without having to increase the dimensions of the existinglandside berm and/or having to move the trench and/or having to tamperwith surrounding structures. The term “larger volumetric weight” refersherein to a (saturated) volumetric weight of the columnmaterial—expressed in kilonewtons per cubic meter (kN/m³)—that is atleast 10%, more specifically at least 25% larger than the volumetricweight(s) of the corresponding soft soil layer(s). The soft soil layersmay correspond to one or more of the soil layers underneath the berm.Alternatively or in addition, the berm may be formed at least partiallyby one or more of such soft soil layers.

The mineral aggregate which serves to increase the total mass of thelandside berm region of the levee may for example comprise crushed rock,gravel, or other mineral granules with a relatively high average massdensity. In one exemplary embodiment an average particle size of themineral aggregate of at least 2 mm and/or to a maximum of 80 mm may beused.

The impermeable filler component in the columns yields a sealing effectand serves to lower a vertical fluid permeability of the columns to suchan extent that the resulting column permeability is similar to or lessthan a fluid permeability of at least the upper soil layer of the berm,through which the columns extend. The columns may for example have afluid permeability that is at least 10% smaller than the permeability ofthe upper soil layer. More specifically, the permeabilities of thecolumns and the upper soil layer may be associated with characteristicsrelating to the transport of water. Hydraulic conductivity (also knownas hydraulic permeability) is a measure of how easily water can passthrough soil or other material. Accordingly, the impermeable fillercomponent in the columns may serve to lower a vertical hydraulicconductivity of the columns to values similar to or less than ahydraulic conductivity of at least the upper soil layer of the bermthrough which the columns extend.

In one example, this upper soil layer may be formed by a clay layer,which typically has a hydraulic conductivity in a range of Kf=10⁻⁵ to10⁻⁷ m/s. In this example, the columns may have a hydraulic conductivityof 10⁻⁷ m/s or smaller.

The smaller permeability (or hydraulic conductivity) of the columns withrespect to the upper soil layer prevents the columns from acting asfluid drainage conduits between the berm surface and the underlying soillayers. Another aspect of using the impermeable filler is that saidfiller material fills in the spaces between the mineral aggregate sothat the overall density and specific weight of the columns isincreased. Thus, the filler also contributes to increase the total massof the landside berm region and the overall stability of the levee.

The columns can comprise for example an amount of at least 60 weightpercent mineral aggregate and/or a maximum of 80 weight percent mineralaggregate. The amount of impermeable filler material can be for exampleat least 20 weight percent and/or can amount to a maximum of 40 weightpercent.

Application of the columns may also yield an increase of the levee'sresistance against the failure mechanism piping, because the leakagelength (i.e. the flow path for the ground water seepage) becomes longer.

Because each of the columns may be inserted into/through the landsideberm and/or heel and/or trench of the levee by means of a localprocedure, and because the resulting column distribution yields anegligible increase in the total size of the levee, the proposed methodmay be employed without needing geometric adaptation of the levee and/orneeding special measures to protect the existing constructions andecology near the levee. In this method, the column distribution may belocally adjusted—possibly even omitting columns in selected regions—toadapt to existing constructions, cabling, conduits, plant roots, etc.The proposed method is particularly effective if applied in conjunctionwith soft soil layers, which have typical volumetric weights rangingfrom 10 kN/m³ to 14 kN/m³. Examples of such soft soil types are peat andorganic clay.

Because columns primarily serve to weight/load the relatively soft soillayers, it is not essential to let the columns extend deeper down into alower supporting soil layer (in the Netherlands, this support layercorresponds to Holocene or at a deeper level the Pleistocene sandlayer). Hence, the columns do not need to be forced into the denselycompacted sand layer e.g. via vibration, so the risk for liquefactioneffects may be kept small.

The levee typically comprises a landside heel that delineates thelandside berm. According to an embodiment of the method, the region inwhich the columns are applied spans at least the berm region directlyadjacent to the landside heel.

According to an embodiment, the impermeable filler comprises a hydraulicbinding agent. The hydraulic binding agent constitutes a binder that isable to harden under water, to allow column formation in the (humid)soil layers in/underneath an existing levee. The binding agent maycomprise at least one of cement, limestone, acrylates, resin and solublesilicates (water glass). This hydraulic binding agent may alsosimultaneously serve as the impermeable filler present in the columns toreduce fluid permeability (or hydraulic conductivity).

As an alternative or in addition, the impermeable filler may alsocomprise a non-binding material such as bentonite. In an exemplaryembodiment, in which the impermeable filler comprises both a hydraulicbinding agent and a non-binding material, an amount of at least 90weight percent of hydraulic binding agent (as percentage of the overallimpermeable filler mass) and/or an amount of a maximum of 10 weightpercent non-binding material (as percentage of the overall impermeablefiller mass) may be used.

According to an exemplary embodiment, the mixture of the aggregatematerial and the impermeable filler may have a volumetric weight with avalue of at least 18 kN/m³. By using a granular mixture of aggregatematerial and impermeable filler, a significance increase of the averagevolumetric weight(s) of the soft soil layer(s) may be achieved, and theconstruction of such columns may be realized by proven methods.

According an embodiment, the granular mixture, respectively the columnsmade therefrom, can comprises further fillers for adapting at least oneof the rheological properties, the strength properties, or the hardeningproperties of the granular mixture and the column, respectively.

Exemplary further fillers are plasticizers that serve to liquefy thefiller suspension, to improve the processability thereof. Other optionsfor the further fillers are retarding agents that serve to delay thestiffening/hardening of the filler suspension, or fiber materials asdiscussed herein below. Preferably, the component of further fillers isrelatively small i.e. up to 5 weight %, and more preferably about 3weight % of the granular mixture.

According to an embodiment, the method comprises determining a slipsurface of most probable sliding fault for the landside slope and thelandside berm, and partitioning of the slip surface into a torqueinducing portion related to the crown of the levee and a torqueresisting portion related to the landside berm. In this embodiment, thecolumns are arranged in the one or more soft soil layers at the torqueresisting portion of the slip surface.

The slip surface of most probable sliding fault may for example be foundby applying Bishop's method on a set of predetermined probable slipcircles. The torque resisting portion in which the columns are placedcorresponds to the passive zone of the slip surface of most probablesliding fault.

According to an embodiment, the method comprises forming the columnswith a first column portion having a first column diameter in an uppersoil layer, and with a second column portion having a second columndiameter in the one or more underlying soil layers. In this embodiment,the second column diameter is at least 30% larger than the first columndiameter.

The column diameter may be kept relatively small in the upper soillayer, which may have a relatively high volumetric weight, whereas thecolumn diameter may be increased further downward in the soil regionthat may correspond to the one or more soft soil layers. Locallyincreasing the column diameter in the soft soil layers allows an optimallocalization of the mass density increase (provided by the columnmaterial) in these soft soil layers, while avoiding excess use of columnmaterial in the soil regions where only little mass density gains can beobtained.

Such columns with locally varying column diameters may be efficientlyobtained by using a depth vibrator for placing the columns into thelandside berm, the landside heel, or the landside trench.

The first and second column portions may form predominantly cylindricalcolumn segments. The first column diameter may for example have a valueof about 0.5 meter, and the second column diameter may for example havea value of at least 0.8 meter.

According to an embodiment, the columns are placed through the landsideberm, the landside heel, and/or the landside trench, and into the one ormore soft soil layers by means of a depth vibrator. The use of a depthvibrator for column construction may have several advantages. Oneadvantage may be that the column material is discharged via the vibratortip, which allows construction of a single continuous column (i.e. asingle body of condensed material, possibly with a uniform or a varyinglateral column diameter). Other advantages of the depth vibratortechnique may be that caving in of the column bore hole may be reducedor avoided, and that there may be no need to use rinsing agents (e.g.water) for keeping the working area accessible. By the use of a depthvibrator, the columns may be positioned into the soil with a provenlocalized and less invasive method, so that disruption of theenvironment may be minimized.

The depth vibrator serves for placing the column material into the soil,thereby replacing and compacting the surrounding soil. The process maythus also be referred to as “vibro replacement” and the depth vibratormay also be referred to as “feed vibrator”. For the construction ofvibro replacement columns a bottom feed process may be used, which feedsthe granular material and impermeable filler to the tip of the vibrator,possibly with the aid of pressurized air. The vibro replacement processconsists of alternating steps. During the retraction step, granularmaterial and filler runs from the vibrator tip into the annular spacecreated and is then compacted and pressed into the surrounding soilduring the subsequent re-penetration step. In this manner the columnsare created from the bottom up, which behave as a composite materialwith the surrounding soil under load.

A potential further advantage of using the depth vibrator techniquerelates to the desired smaller permeability (or hydraulic conductivity)of the columns with respect to the upper soil layer. A local compactingeffect of the depth vibrator onto the soil layers during placement of acolumn may cause these local soil portions to tightly envelop andseamlessly engage with the resulting column. Tight column envelopment bythe upper soil layer may avoid creation of an annular gap directlyaround the column, which in turn helps to reduce or even avoid verticalfluid drainage between the berm surface and the underlying soil layers.

According to a further embodiment, the method comprises drilling a firstvoid portion and removing soil from the upper soil layer, prior toplacing the columns through the landside berm into the one or more softsoil layers with the depth vibrator.

By drilling the first void portion and removing the corresponding soilprior to forming the corresponding column, the probability of negativesoil displacement effects will be reduced. This further reduces theprobability of disrupting directly surrounding structures (likefoundations, cabling, conduits, and plant roots) during columnplacement.

According to an embodiment, placing of the columns comprises arrangingat least two columns at transversally consecutive positions across thelandside berm (i.e. consecutive positions with respect to thetransversal coordinate).

The transversal direction corresponds to the landside direction. Thearrangement of two or more columns (or even two or more longitudinalcolumn rows) at transversally consecutive positions along the landsideberm ensures that the transversal extent of the landside berm isutilized for column placement, to achieve the desired increases in massdensity and stabilization.

According to an embodiment, the method comprises placing the columnswith a predominantly vertical orientation in the one or more soft soillayers.

The proposed method allows stability to be increased by using justvertical columns. The use of vertical columns poses less severerequirements to the equipment needed for constructing and placing thecolumns. Moreover, the shape of columns can be controlled effectively,in contrast to tilted columns.

According to an embodiment, the method comprises placing the columns ina spatially separated distribution in the one or more soft soil layers.

The term “spatially separated” refers herein to a spatial distributionof the columns in the longitudinal direction and the transversaldirection, such that the columns do not mutually touch or overlap.Preferably, sufficient spacing is left open between the columns, suchthat at least 50% of the cross sectional profile viewed along thetransversal direction remains available for the soil and for watercurrents flowing there through. This significantly reduces theprobability of causing hydrological blockades. According to a furtherembodiment, the method comprises placing the columns with mutual spacingwith a value in a range of 0.5 meter to 2.5 meter, preferably withmutual spacing in a range of 1.0 meter to 2.0 meter, and more preferablyof about 1.5 meter.

According to an embodiment, a supporting sand layer is situatedunderneath the soft soil layers, and the columns are arranged so as toextend through the one or more soft soil layers into the supporting sandlayer. In this embodiment, the columns comprise reinforcing material forincreasing a strength and/or stability of the columns.

By letting the columns extend deeper into the lower supporting sandlayer, shear strains that occur along the sliding surface may betransferred via the columns to the supporting sand layer. The columnsmay function as so-called “dowels”, and the resulting increase of theshear resistance in the soft soil layers will yield an additionalincrease in the inwards macro stability of the levee.

According to a further embodiment, the reinforcing material may comprisefiber materials.

According to a further embodiment, the reinforcing material can comprisea geo textile for enveloping the columns, and the method may compriseapplying the geo textile prior to placing the columns.

According to another further embodiment, the reinforcing material may beformed as steel rods, and wherein the method may comprise inserting thesteel rods in the columns after placing the columns.

By applying reinforcing steel and/or geo textile, the shear strength ofthe columns will be increased. This may lead to a further improvement ofthe inwards macro stability for the levee, or to a reduction of thenumber of columns or the size of the column area needed to achieve apredetermined macro stability.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying schematic drawings in which correspondingreference symbols indicate corresponding parts, and in which:

FIG. 1 schematically shows a cross-sectional view with typical elementsof a levee according to the prior art;

FIG. 2 presents a cross-sectional view of a portion of a levee and aslip surface according to an embodiment of the invention;

FIG. 3 shows a column distribution in a landside berm according to anembodiment;

FIG. 4 shows a column distribution in a landside berm, a landside heel,and a landside trench according to another embodiment, and

FIG. 5 illustrates a method for applying stabilization columns accordingto a method embodiment.

The figures are meant for illustrative purposes only, and do not serveas restriction of the scope or the protection as laid down by theclaims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following is a description of certain embodiments of the invention,given by way of example only and with reference to FIGS. 2-4.

Embodiments according to the invention generally relate to methods forimproving a stability of an existing levee 10. The examples presentedbelow will therefore be discussed with reference to the generalterminology and reference numbers discussed herein above with referenceto FIG. 1.

FIG. 2 illustrates that the levee 10 and the subsoil elements 16-20 maytypically be formed by soil layers 38-46 of different constitution andtypical mass densities. An first (uppermost) soil layer 38 may consistessentially of clay, whereas lower subsequent soil layers mayessentially consist of peat 40, clay 42, sand clay 44, and a supportingsand layer 46 respectively. For levees in general, the hydrologicalproperties and the response of such soil layers 38-46 to changing waterconditions influence the levee's stability. In particular, the peatlayer 40 has a high fluid absorbing capacity and a low (unsaturated)mass density compared to the other soil layers. Changing waterconditions (e.g. the height of the water body 35 on the water side 12)affect the mechanical properties and fluid dynamics in the soil layers38-46, and the peat layer 40 in particular. This is highly riskful forfoundations in these areas.

The proposed method comprises applying columns 60 through the existinglandside berm 32 and/or heel 33 and/or trench 36 into one or more softsoil layers 40 (and possibly 38 and 42-46, depending on local soilconditions), which are situated directly below the levee 10, and whichhave corresponding volumetric weight γs. The columns 60 have a columnvolumetric weight γc that is significantly larger than the volumetricweights γs of the soft soil layer 40 (and possibly 38, 42-46). Thecolumns 60 serve as mass loading structures, intended to increase atotal mass of a torque resisting portion 54 of the land side 14 of thelevee 10, which corresponds with the landside berm 32. The applicationof such columns 60 may also reduce the average leakage length forseepage underneath the levee 10, and hence may lower the risk for pipingeffects.

In this exemplary embodiment, the soft soil layer(s) is formed by a peatlayer 40 with a volumetric weight γp having a typical value in a rangeof 9 kN/m³ to 14 kN/m³. In contrast, the columns 60 have a volumetricweight γc of at least 18 kN/m³. The columns 60 are positioned extendingat least down to the soft soil layer(s) (e.g. peat layer) 40, toincrease the average volumetric weight in this layer. Note that inalternative embodiments, any number of soft soil layers may be presentin the levee and any ordering of the layers may be possible. Forexample, a levee may be formed on a soil layer configuration withmultiple soft peat layers and intermediate denser soil layers.

In this example, the method comprises determining a slip surface 50 ofmost probable sliding fault for the landside slope 28 and the landsideberm 32 of the levee 10.

This slip surface 50 is subsequently partitioned into a torque inducingportion 52 situated near the crown 24 of the levee 10 on the one hand,and a torque resisting portion 54 near the land side berm 32. Thesesurface portions 52, 54 are separated by a boundary line 56.The columns60 are then arranged in the soft soil layer(s) 40 (38, 42) at the torqueresisting portion 54 of the slip surface 50.

This slip surface 50 of most probable sliding fault may for example befound by applying Bishop's method on a set of predetermined probableslip circles corresponding with a distribution of adjacent circlecenters 58. Sliding fault assessment may also be possible withalternative slip surface calculation methods or finite element methods.In an embodiment wherein Bishop's method is used, the slip surface 50 ofmost probable sliding fault is mapped to a slip circle 50 with a circlecenter 58 a. The torque resisting portion 54 corresponds in this casewith the passive zone of the slip circle 50 of most probable slidingfault. The soft soil layer(s) 40 (38, 42-46) will thus be locallyweighted within the torque resisting portion 54 of the slip surface 50,so that the torque component with respect to the circle center 58 a ofthe slip surface 50—which counteracts the torque from the soil in thetorque inducing portion 52—is significantly increased. This torquecompensation yields an increased stability for the levee 10.

Due to the presence of the columns 60 in the torque resisting portion 54of the slip surface 50, the average shear strength of the soft soillayer(s) 40 (38, 42) may also be locally increased in this region,yielding a further improvement of the stability factor.

FIG. 3 depicts a cross-section of a column distribution in the landsideberm 32, according to an embodiment. The columns 60 have been arrangednear the landside heel 33, in such a manner that the local increase ofthe (average) volumetric weights of the clay layer 38 and the peat layer40 extends inwards towards the land side 14. As a result, the countertorque generated by the torque resisting portion 54 is significantlyincreased. Because natural material is not removed or remolded butpushed together, the natural structure of peat is preserved, prohibitingloss of shear strength.

In this example, the columns 60 consist essentially of a granularmixture of gravel and cement-bentonite, with further fillers foradjusting the rheological properties of the cement-bentonite suspension,and the strength and hardening properties of the resulting granularmixture. The following global component ratio may be adhered to: about80 weight % gravel and about 20 weight % fillers. Such granular mixturemay have a typical volumetric weight with a value of about 20 kN/m³. Thecolumns 60 are vertically impermeable for reasons of water safety. Thephrase “vertically impermeable” implies in this context that thewater/fluid permeability (hydraulic conductivity) of the columns issimilar to or even less than a permeability (hydraulic conductivity) ofthe surrounding upper soil layer 38. As a result, the columns 60 are notformed to act as vertical fluid drainage conduits.

As is shown in FIG. 3, at least a portion of the columns 60′ may extendthrough the soft soil layers 40, and down to the underlying soil layers42, 44 as well as the supporting sand layer 46. This column portion 60′provides a dowel effect, that allows transferring of sliding loadsoccurring along sliding surfaces (e.g. on the slip surface 50) via thiscolumn portion 60′ to the supporting sand layer 46. Through addition offibrous material to the granular mixture, the shear strength of theresulting column portion 60′ will be increased, which may improve thelocal (average) shear resistance of the soft soil layer(s) 40 (38,42-46) in the torque resisting portion 54.

According to FIG. 3, the columns 60, 60′ in this example are formed withcolumn sections 62-66 that have different characteristic cross-sectionaldimensions Ø1-Ø3. Each of the columns 60, 60′ is accommodated in acolumn void (not indicated) that extend through at least some of thesoil layers 38-46, and which includes at least and upper void section72. Such columns 60, 60′ may be formed via a method described furtherbelow. In this example, the column sections 62-66 have (macroscopically)cylindrical shapes. Here, the diameters Ø2 of the second column sections64 located in the peat layer 40 have the largest value, to achieve asignificant increase in the average volumetric weight of the (relativelylight) peat layer 40. In this example, the first column sections 62 inthe upper clay layer 38 have upper diameters Ø1 of about 0.5 meter, andthe second column sections 64 in the peat layer 40 have diameters Ø2 ofabout 0.8 meter. A lowest diameter Ø3 of the third column sections 66(which belong to the columns 60′ that extend down to the supporting sandlayer 46) is comparable to the upper diameters Ø1 i.e. about 0.5 meter.

In this example, the columns 60, 60′ have been positioned in a mutuallyseparated arrangement (viewed along the longitudinal and transversaldirections X,Y), such that the columns 60, 60′ do not mutually touch oroverlap. The column distribution in this example may be characterized bya transversal nearest neighbor distance ΔY of about 1.0 meter to 1.5meter, and a longitudinal nearest neighbor distance of about 1.5 meter.

FIG. 4 shows a cross-sectional view of levee with a distribution ofcolumns in a landside berm 132, a landside heel 133, as well as alandside trench 136. Features in the levee embodiment described abovewith reference to FIG. 3 are also be present in the levee embodimentshown in FIG. 4, and will not all be discussed here again. For thediscussion with reference to FIG. 4, like features are designated withsimilar reference numerals preceded by 100, to distinguish theembodiments.

In this embodiment, additional columns 160′, 160″ with associated widersecond column sections 164′, 164″ have been placed directly through thelandside heel 133 and through the bottom of the trench 136. The local(average) volumetric weights of the clay layer 138 and the peat layer140 is therefore increased in these regions as well, yielding a furtherincrease of the counter torque generated by the torque resistingportion.

FIG. 5 illustrates a method for forming and placing the columns 60, 60′in the soft soil layer(s) 40 (38, 42-46) according to an embodiment. Inthis example, a depth vibrator 100 is employed for in-situ constructionof the columns 60, 60′. An exemplary depth vibrator is described inpatent document EP1234916. In the example of FIG. 5, the depth vibrator100 is attached to a rig 106, and comprises a supply tube 102 with adischarge opening 103 at a lower end thereof, a suspension conduit 104for supplying a filler suspension to the supply tube 102, and a trough105 for locally supplying gravel. The supply tube 102 is rotatable withrespect to an axis along the height direction Z. The supply tube 102 isrotatable in an eccentric manner about this axis (i.e. the symmetry axisand rotation axis of the supply tube 102 are parallel but laterallyshifted), to enable the depth vibrator 100 to force soil componentslaterally away while the supply tube 102 is lowered into the varioussoil layers 38-46.

The depth vibrator 100 and rig 106 require only a single deployment,after which the supply tube 102 may be lowered into the soil layers38-46 of the existing landside berm 32. The eccentric rotation of thesupply tube 102 about its axis of rotation causes local lateraldisplacement of the soil and allows penetration of the supply tube 102with its discharge opening at least down to the peat layer 40. Toconstruct the extended columns 60′, the depth vibrator needs to befurther lowered with its discharge opening 103 down into the furtherlayers 42-46, and down to the supporting sand layer 46 in particular.

Subsequently, gravel is provided into the trough 105 (e.g. via a powershovel). The gravel and the filler suspension are allowed to mix insidethe supply tube 102. Once the depth vibrator 100 has reached itsintended depth, the supply tube 102 is slightly lifted, which allows thegranular mixture to emanate from the discharge opening 103 into thelocally created column void. The depth vibrator 100 is intermittentlylowered in a vibrating manner, to locally condense the granular mixtureand laterally force the mixture into the respective soil layer 38-46.Via intermittent raising and lowering of the depth vibrator 100, acontinuous column 60, 60′ is formed that extends through the soil layers38-46 up to a desired height.

In one embodiment, the depth vibrator 100 has a maximum tube diameter Øtof 0.5 meter. When such a depth vibrator 100 is urged though thesupporting sand layer 46, the first local column diameter Ø1 will have asimilar value of about 0.5 meters. Due to the weak soil cohesion of thepeat layer 40, the second local column diameter Ø2 in this peat layer 40will be considerably larger e.g. about 0.8 meter or larger.

In order to reduce the probability of negative soil displacement effectsin the upper clay layer 38, first void sections 72 may initially bedrilled and the corresponding clay removed, before the columns 60, 60′are formed in the soil layers 38-46 by means of the depth vibrator 100.Alternative measures may be variations of patterns and diameter aroundcritical objects.

The descriptions above are intended to be illustrative, not limiting. Itwill be apparent to the person skilled in the art that alternative andequivalent embodiments of the invention can be conceived and reduced topractice, without departing from the scope of the claims set out below.

LIST OF REFERENCE SYMBOLS

10 levee

12 water side

14 land side

16 waterside land

18 lowland

20 levee base

22 levee core

24 crown

26 waterside slope

28 landside slope

30 waterside berm

31 waterside toe

32 landside berm

33 landside heel

35 water body

36 drainage trench

38 first soil layer e.g. clay

40 second soil layer e.g. peat

42 third soil layer e.g. clay

44 fourth soil layer

46 fifth soil layer e.g. Pleistocene or Holocene sand

48 berm sand

50 slip surface (e.g. circle)

52 torque inducing portion

54 torque resistive portion

56 boundary line

58 set of circle centers

60 column (with mineral aggregate and impermeable filler)

62 first column section

64 second column section

66 third column section

68 fiber material

70 column void

72 first void section

100 depth vibrator

102 supply tube

103 discharge opening

104 suspension supply conduit

105 trough

106 rig

Ø1 first column diameter

Ø2 second column diameter

Ø3 second column diameter

Øt vibrator diameter

X longitudinal direction

Y transversal direction

Z height direction

What is claimed is:
 1. A method for improving an inwards stability of anexisting levee including a landside slope and at least one of a landsideberm, a landside heel, and a landside trench, wherein the methodcomprises: placing columns through at least one of the landside berm,the landside heel, and the landside trench into one or more soft soillayers with corresponding soil volumetric weights, wherein the columnshave a column volumetric weight that is larger than the soil volumetricweights, and wherein the columns comprise a mineral aggregate and animpermeable filler.
 2. The method according to claim 1, wherein thecolumn volumetric weight of the columns is at least 10% larger than thesoil volumetric weights of the one or more soft soil layers.
 3. Themethod according to claim 1, wherein the impermeable filler comprises ahydraulic binding agent.
 4. The method according to claim 1, comprising:determining a slip surface of most probable sliding fault for thelandside slope and the landside berm; partitioning the slip surface intoa torque inducing portion related to the crown of the levee and a torqueresisting portion related to the landside berm; wherein placing thecolumns involves arranging the columns in the one or more soft soillayers at the torque resisting portion.
 5. The method according to claim1, comprising: forming the columns with a first column portion having afirst column diameter in an upper soil layer, and with a second columnportion having a second column diameter in the one or more underlyingsoil layers, wherein the second column diameter is larger than the firstcolumn diameter.
 6. The method according to claim 5, wherein the secondcolumn diameter is at least 30% larger than the first column diameter.7. The method according to claim 1, wherein the columns are placedthrough at least one of the landside berm, the landside heel, and thelandside trench, and into the one or more soft soil layers by means of adepth vibrator.
 8. The method according to claim 7, comprising: drillinga first void portion and removing soil from the upper soil layer, priorto placing the columns through at least one of the landside berm, thelandside heel, and the landside trench into the one or more soft soillayers with the depth vibrator.
 9. The method according to claim 1,wherein placing the columns comprises arranging at least two columns attransversally consecutive positions across the landside berm.
 10. Themethod according to claim 1, comprising: placing the columns with apredominantly vertical orientation in the one or more soft soil layers.11. The method according to claim 1, comprising: placing the columns ina spatially separated distribution in the one or more soft soil layers.12. The method according to claim 11, comprising: placing the columnswith mutual spacing with a value in a range of 0.5 meter to 2.5 meter,preferably with mutual spacing in a range of 1.0 meter to 2.0 meter, andmore preferably of about 1.5 meter.
 13. The method according to claim 1,wherein the columns comprise further fillers for adapting at least oneof the rheological properties, the strength properties, or the hardeningproperties of the columns.
 14. The method according to claim 1, whereina supporting sand layer is situated underneath the soft soil layers,wherein the columns are arranged so as to extend through the one or moresoft soil layers into the supporting sand layer, and wherein the columnscomprise reinforcing material for increasing a strength and/or stabilityof the columns.
 15. The method according to claim 14, wherein thereinforcing material comprises fiber materials.
 16. The method accordingto claim 14, wherein the reinforcing material comprises a geo textilefor enveloping the columns, and wherein the method comprises: placingthe geo textile prior to placing the columns.
 17. The method accordingto claim 14, wherein the reinforcing material is formed into steel rods,and wherein the method comprises: inserting the steel rods into thecolumns after placing the columns.
 18. A method for improving an inwardsstability of an existing levee including a landside slope and at leastone selected from a group consisting of a landside berm, a landsideheel, and a landside trench, wherein the method comprises: placingcolumns through at least one selected from the group consisting of thelandside berm, the landside heel, and the landside trench, into one ormore underlying soft soil layers with corresponding soil volumetricweights, wherein the columns have a column volumetric weight that is atleast 10% larger than the soil volumetric weights, and wherein thecolumns comprise a mineral aggregate and an impermeable filler. formingthe columns with a first column portion having a first column diameterin an upper soil layer, and with a second column portion having a secondcolumn diameter in the one or more underlying soil layers, wherein thesecond column diameter is at least 30% larger than the first columndiameter.